PROGRAMA DE MONITOREO

When you were just a 2-week-old embryo, your very existence still unknown to your parents, cells from the embryo’s upper surface began to form a sheet that rearranged itself by turning inward and curling into a neural tube. This phenomenon, called neurulation, signaled the beginning of your central nervous system’s development. Once formed, this structure was covered over by another sheet of cells, to become your skin, and was moved inside you so that the rest of your body could develop around it. Around the 25th day of your gestational life, your neural tube began to take on a pronounced curved shape. At the top of your “C-shaped” embryonic self, three distinct bulges appeared, which eventually became your hindbrain, midbrain, and forebrain. (See Figure 2.6.)

Good prenatal and postnatal nutrition is essential for chil- dren to reach their full cogni- tive potential.

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Within the primitive neural tube, important events were occurring. Cells from the interior surface of the neural tube reproduced to form neurons, or nerve cells, that would become the building blocks of your brain. From about the 40th day, or 5th week, of gestation, your neurons began to increase at a staggering rate—one quarter of a million per minute for 9 months—to create the 100 billion neurons that make up a baby’s brain at birth. At least half would be destroyed later either because they were unnecessary or were not used. We will have more to say about this loss of neurons later.

Your neurons began to migrate outward from their place of birth rather like filaments extending from the neural tube to various sites in your still incomplete brain. Supporting cells called glial cells, stretching from the inside of the neural tube to its outside, provided a type of scaffolding for your neurons, guiding them as they ventured out on their way to their final destinations. Those neurons that devel- oped first migrated only a short distance from your neural tube and were destined to become the hindbrain. Those that developed later traveled a little farther and ultimately formed the midbrain. Those that developed last migrated the farthest to populate the cerebral cortex of the forebrain. This development always pro- gressed from the inside out, so that cells traveling the farthest had to migrate through several other already formed layers to reach their proper location. To build the six layers of your cortex, epigenetic processes pushed each neuron toward its ultimate address, moving through the bottom layers that had been already built up before it could get to the outside layer. (See Box 2.1 and Figures 2.7 and 2.8.)

35 Days 25 Days

Four Months Six Months Seven Months

Eight Months

40 Days 50 Days 100 Days

Nine Months

FIGURE 2.6 Stages of brain development. The developing human brain viewed from the side in a succession of embryonic and fetal stages. The small figures beneath the first five embryonic stages are in proper relative proportion to the last five figures.

Source: cowan, W. M. (1979). The development of the brain. In r. r. Llinas (ed.). The workings of the

brain (p. 41). Scientific American (Sept. 1979), p. 166. Illustration by Tom Prentiss. reproduced with

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Multidimensional models of mental health and psychopathology now incorporate genetics and brain processes into their conceptual frameworks. Thus, a working knowledge of the brain and its func- tioning should be part of a contemporary helper’s toolkit. consumers of research also need this background to understand studies that increasingly include brain-related measures. Here we present a very short introduction to some important brain areas and describe their related functions.

The complex human brain can be partitioned in various ways. one popular way identifies three main areas that track evolution- ary history: hindbrain, midbrain, and forebrain. Bear in mind, how- ever, that brain areas are highly interconnected by neural circuitry despite attempts to partition them by structure or function. In gen- eral, the more complex, higher-order cognitive functions are served by higher-level structures while lower-level structures control basic functions like respiration and circulation.

Beginning at the most ancient evolutionary level, the hindbrain structures of medulla, pons, cerebellum, and the reticular forma- tion regulate autonomic functions that are outside our conscious control. The medulla contains nuclei that control basic survival func- tions, such as heart rate, blood pressure, and respiration. Damage to this area of the brain can be fatal. The pons, situated above the me- dulla, is involved in the regulation of the sleep–wake cycle. Individu- als with sleep disturbances can sometimes have abnormal activity in this area. The medulla and the pons are also especially sensitive to an overdose of drugs or alcohol. Drug effects on these structures can cause suffocation and death. The pons transmits nerve impulses to the cerebellum, a structure that looks like a smaller version of the brain itself. The cerebellum is involved in the planning, coordination, and smoothness of complex motor activities such as hitting a tennis ball or dancing, in addition to other sensorimotor functions.

Within the core of the brainstem (medulla, pons, and midbrain) is a bundle of neural tissue called the reticular formation that runs up through the midbrain. This, together with smaller groups of neu- rons called nuclei, forms the reticular activating system, that part of the brain that alerts the higher structures to “pay attention” to incoming stimuli. This system also filters out the extraneous stimuli that we perceive at any point in time. For example, it is possible for workers who share an office to tune out the speech, music, or gen- eral background hum going on around them when they are involved in important telephone conversations. However, they can easily “perk up” and attend if a coworker calls their name.

The midbrain also consists of several small structures (superior colliculi, inferior colliculi, and substantia nigra) that are involved in vision, hearing, and consciousness. These parts of the brain re- ceive sensory input from the eyes and ears and are instrumental in controlling eye movement.

The forebrain is the largest part of the brain and includes the cerebrum, thalamus, hypothalamus, and limbic system struc- tures. The thalamus is a primary way station for handling neural communication, something like “information central.” It receives information from the sensory and limbic areas and sends these

messages to their appropriate destinations. For example, the thalamus pro jects visual information, received via the optic nerve, to the occipital lobe of the cortex (see below). on both sides of the thalamus are structures called the basal ganglia. These structures, especially the nucleus accumbens, are involved in motivation and approach behavior (Galvan et al., 2006).

The hypothalamus, situated below the thalamus, is a small but important area that regulates hunger, thirst, body temperature, and breathing rate. Lesions in areas of the hypothalamus have been found to produce eating abnormalities in animals, including obesity (Leibowitz, Hammer, & chang, 1981) or starvation (Anand & Brobeck, 1951). It is also important in the regulation of emotional, stress- related responses. The hypothalamus functions as an intermediary, translating the emotional messages received from the cortex and the amygdala into a command to the endocrine system to release stress hormones in preparation for fight or flight. We will discuss the hypothalamus in more detail in the section on the body’s stress systems.

limbic structures (hippocampus, amygdala, septum, and cin- gulate cortex) are connected by a system of nerve pathways (limbic system) to the cerebral cortex. often referred to as the “emotional brain,” the limbic system supports social and emotional functioning and works with the frontal lobes of the cortex to help us think and reason. The amygdala rapidly assesses the emotional significance of environmental events, assigns them a threat value, and conveys this information to parts of the brain that regulate neurochemical func- tions. The structures of the limbic system have direct connections with neurons from the olfactory bulb, which is responsible for our sense of smell. It has been noted that pheromones, a particular kind of hormonal substance secreted by animals and humans, can trigger particular reactions that affect emotional responsiveness below the level of conscious awareness. We will have more to say about the workings of the emotional brain and its ties to several emotional disorders in chapter 4.

other limbic structures, notably the hippocampus, are critical for learning and memory formation. The hippocampus is especially important in processing the emotional context of experience and sensitive to the effects of stress (Fink, 2009). under prolonged stress, hippocampal neurons shrink and new neurons are not produced (Sapolsky, 1984). The hippocampus and the amygdala are anatomi- cally connected, and together they regulate the working of the HPA axis (described later in this chapter). In general, the amygdala acti- vates this stress response system while the hippocampus inhibits it (Mcewen & Gianaros, 2010).

The most recognizable aspect of the forebrain is the cerebrum, which comprises two thirds of the total mass. A crevice, or fissure, divides the cerebrum into two halves, like the halves of a walnut. Information is transferred between the two halves by a network of fibers comprising the corpus callosum. These halves are referred to as the left and right hemispheres. research on hemispheric spe- cialization (also called lateralization), pioneered by Sperry (1964), demonstrated that the left hemisphere controls functioning of the

Box 2.1: The Major Structures of the Brain

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Forebrain Midbrain Hindbrain Medulla Pituitary gland Hypothalamus Reticular formation Part of the hindbrain where

nerves cross from one side of the body to the opposite side of the brain.

Part of the hindbrain that relays messages between the cerebellum and the cortex.

A system of nerves, running from the hindbrain and through the midbrain to the cerebral cortex, controlling arousal and attention.

Cerebellum

Part of the hindbrain that controls balance and maintains muscle coordination.

Thalamus Corpus callosum

Part of the forebrain that relays information from sensory organs to the cerebral cortex. Connects left and right hemispheres of the brain.

Part of the forebrain that regulates the amount of fear, thirst, sexual drive, and aggression we feel.

Pons Hippocampus Amygdala Hindbrain, Midbrain, and Forebrain

Limbic system

Plays a role in our emotions, ability to remember, and ability to compare sensory information to expectations.

Influences our motivation, emotional control, fear responses, and interpretations of nonverbal, emotional expressions. A group of brain structures

in the forebrain that affects our emotions.

Cerebral cortex

Controls complex thought processes.

This is a picture of the brain from the left side. Beneath the cortex are three parts of the brain: the hindbrain, the midbrain, and the forebrain. The hindbrain controls some basic processes necessary for life. The midbrain is a relay station to the brain. The forebrain is where complex thoughts, motives, and emotions are stored and processed.

Regulates other endocrine glands.

FIGURE 2.7 The major structures of the brain.

Source: uba, L. & Huang, K. (1999). Psychology. reprinted by permission of Laura uba. right side of the body and vice versa. Language functions such as vo-

cabulary knowledge and speech are usually localized in the left hemi- sphere, and visual–spatial skills are localized on the right. recently, this research was introduced to lay readers through a rash of popular books about left brain–right brain differences. overall, many of these publications have distorted the facts and oversimplified the findings. Generally the hemispheres work together, sharing information via the corpus callosum and cooperating with each other in the execu- tion of most tasks (Geschwind, 1990). There is no reliable evidence that underlying modes of thinking, personality traits, or cultural differ- ences can be traced exclusively to hemispheric specialization.

each hemisphere of the cerebral cortex can be further divided into lobes, or areas of functional specialization (see Figure 2.8). The occipital lobe, located at the back of the head, handles visual infor- mation. The temporal lobe, found on the sides of each hemisphere, is responsible for auditory processing. At the top of each hemi- sphere, behind a fissure called the central sulcus, is the parietal

lobe. This area is responsible for the processing of somatosensory information such as touch, temperature, and pain. Finally, the fron- tal lobe, situated at the top front part of each hemisphere, controls voluntary muscle movements and higher level cognitive functions.

The prefrontal cortex (pFC) is that part of the frontal lobe that occupies the front or anterior portion. This area is involved in pro- cesses like sustained attention, working memory, planning, decision- making and emotion-regulation. Generally, the PFc plays a role in regulation and can moderate an overactive amygdala as well as the activity of the HPA axis. Another important regulatory pathway in- volves the anterior cingulate cortex (ACC), a structure in the middle of the brain above the corpus callosum. The Acc mediates cognition and affect. Impaired connections between the Acc and the amygdala are related to higher levels of anxiety and neuroticism, and lower Acc volume has been found in depressed patients (Lopez-Munoz & Alamo, 2011). The size of the various brain regions and the integrity of their circuitry play a role in individuals’ cognition, affect, and behavior.

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Scientists have discovered that neurons sometimes need to find their destina- tions (for example, on the part of the cortex specialized for vision) before that part of the cortex develops. It’s a little like traveling in outer space. Or as Davis (1997) has suggested, “It’s a bit like an arrow reaching the space where the bull’s-eye will be before anyone has even set up the target” (pp. 54–55). Certain cells behave like signposts, providing the traveling neurons with way stations as they progress on their journey. Neurons may also respond to the presence of certain chemicals that guide their movements in a particular direction.

About the 4th month of your prenatal life, your brain’s basic structures were formed. Your neurons migrated in an orderly way and clustered with similar cells into distinct sections or regions in your brain, such as in the cerebral cortex or in the specific nuclei. The term nucleus here refers to a cluster of cells creating a structure, rather than to the kind of nucleus that is found in a single cell. An example is the nucleus accumbens, part of the basal ganglia in the brain’s interior.

As we have seen, one important question concerns just how specialization of cells in different regions of the brain occurs and what directs it. This issue is still controversial and extraordinarily complicated to research. However, most available evidence supports the view that cortical differentiation is an epigenetic process, primarily influenced by the kinds of environmental inputs the cortex receives. In other words, the geography of the cortex is not rigidly built in but responds to activity and experiences by making changes in its structural organization. This principle was demonstrated by researchers who trans- planted part of the visual cortex of an animal to its parietal lobe (O’Leary & Stanfield, 1989). The transplanted neurons began to process somatosensory rather than visual information. Studies such as these have shown that the brain is amazingly malleable and demonstrates great neuroplasticity, particularly during early stages of development. In time, however, most cells become specialized for their activity, and it is harder to reverse their operation even though neuroplasticity continues to exist throughout life.

In document I. DATOS GENERALES. 2. Nombre del Proyecto: El proyecto se denomina "Autopista El Salto-Concordia ". (página 85-91)